Regulation of ribosomal RNA synthesis and its bearing on the bobbed phenotype in Drosophila melanogaster

The rate of ribosomal RNA (rRNA) synthesis was studied both in vitro and in vivo using ovaries from Drosophila melanogaster stocks carrying different amounts of DNA complementary to ribosomal RNA (rDNA). It was shown that the rate of rRNA synthesis is reduced in flies exhibiting the bobbed phenotype and having an rDNA content lower than 0.130%. These flies have been shown to lay eggs at a reduced rate but of normal RNA content. Ovaries from phenotypically wild flies, with rDNA contents of 0.180%, 0.370%, and 0.580% showed instead about the same rate of rRNA synthesis. Females with such rDNA contents lay eggs of the same RNA content and at the same rate. These data allowed us to postulate the existence of regulatory mechanisms for rRNA synthesis. In spite of the reduced rate of synthesis in certain tissues, bobbed flies (both males and females) have been shown to have the same RNA/DNA ratio as phenotypically wild flies with different rDNA contents. The same results have been obtained using an intermediate stage (pupae). It is known that bobbed flies exhibit a delayed development. Probably this allows for a normalization of the rRNA content. The genesis of the bobbed phenotype is discussed on the basis of the results obtained.